A buffer solution contains 0.496 M hydrocyanic acid and 0.399 M sodium cyanide . If 0.0461 moles of sodium hydroxide are added to 225 mL of this buffer, what is the pH of the resulting solution ? (Assume that the volume does not change upon adding sodium hydroxide. )

Answer : The pH of the solution is, 9.63

Explanation : Given,

The dissociation constant for HCN =

First we have to calculate the moles of HCN and NaCN.

and,

The balanced chemical reaction is:

Initial moles     0.1116       0.0461     0.08978

At eqm.       (0.1116-0.0461)    0       (0.08978+0.0461)

                        0.0655                       0.1359

Now we have to calculate the pH of the solution.

Using Henderson Hesselbach equation :

Now put all the given values in this expression, we get:

Therefore, the pH of the solution is, 9.63

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